Improve support for external keyboards.

Use Vendor ID, Product ID and optionally the Version to
locate keymaps and configuration files for external devices.

Moved virtual key definition parsing to native code so that
EventHub can identify touch screens with virtual keys and load
the appropriate key layout file.

Cleaned up a lot of old code in EventHub.

Fixed a regression in ViewRoot's fallback event handling.

Fixed a minor bug in FileMap that caused it to try to munmap
or close invalid handled when released if the attempt to map
the file failed.

Added a couple of new String8 conveniences for formatting strings.

Modified Tokenizer to fall back to open+read when mmap fails since
we can't mmap sysfs files as needed to open the virtual key
definition files in /sys/board_properties/.

Change-Id: I6ca5e5f9547619fd082ddac47e87ce185da69ee6